The present invention relates to a rare earth oxide powder and a method for the preparation thereof. More particularly, the invention relates to a rare earth oxide powder having a unique morphological configuration of particles and specific particle size as well as particle size distribution and a method for the preparation of such a rare earth oxide powder by the procedure of calcination of a thermally decomposable salt of the rare earth element.
As is well known, oxides of rare earth elements are industrially important materials used, for example, as a starting material of the red-emitting phosphors in fluorescent lamps and color cathode-ray tubes of television sets, sintering aid in high-performance ceramics and so on.
The most typical method for the preparation of a rare earth oxide powder is calcination of a thermally decomposable salt of the rare earth element. Examples of the thermally decomposable salt of a rare earth element used in this process include oxalate of the rare earth element, which is obtained by mixing an aqueous solution of a water-soluble inorganic salt, e.g., chloride and nitrate, of the element and oxalic acid either as such or in the form of an aqueous solution to precipitate the oxalate, and rare earth ammonium oxalate of the formula NH.sub.4 (RE)(C.sub.2 O.sub.4).sub.2.nH.sub.2 O, in which RE is an atom of the rare earth element, which is obtained by mixing an aqueous solution of a water-soluble inorganic salt of the rare earth element and an aqueous solution of oxalic acid in the presence of ammonia (see, for example, Journal of Inorganic and Nuclear Chemistry, volume 26, page 931, 1964, and Acta Cryst., volume 23, page 949, 1967). A rare earth ammonium oxalate can also be obtained by the reaction of a rare earth hydroxide with an ammoniacal oxalic acid solution or by the reaction of a strongly acidic solution of a rare earth oxalate with an aqueous ammonia solution and the process of thermal decomposition of the rare earth ammonium oxalate into oxide was studied and reported (see Bulletin of Chemical Society of Japan, volume 63, page 378 and page 2115, 1990). These methods are of course applicable to the preparation of an oxide powder of two kinds or more of the rare earth elements in combination as in the preparation of an europoium-doped yttrium or gadolinium oxide powder used as the base material of red-emitting phosphors.
One of the problems in the above described methods is that the particle configuration and size of the thus prepared rare earth oxide particles are uncontrollably very irregular and uneven so that, when a phosphor is prepared from such a rare earth oxide powder, the particle configuration of the phosphor is also irregular and uneven resulting in unevenness of the phosphor layer formed by coating with a phosphor-containing composition and hence unevenness of the brightness of the phosphor-coated surface under excitation. In addition, the particle size of conventional rare earth oxide powders is generally too fine as a starting material for the preparation of phosphors. A rare earth oxide powder of irregular particle configuration is also less effective as a sintering aid when it is used in the preparation of ceramic materials due to the poor dispersibility thereof. The inventors have conducted extensive investigations to solve this problem arriving at a discovery that an oxide powder can give quite satisfactory results when the particles of the oxide powder have a polyhedral particle configuration with a narrow particle size distribution.
When a rare earth ammonium oxalate is washed with water and then calcined into an oxide powder, a part of the rare earth oxide particles have a cubic or polyhedral configuration but, in this process, the fraction of the cubic or polyhedral particles cannot be large enough with a relatively small particle size. In addition, the reproducibility in respect of the particle configuration and particle size of the rare earth oxide powder cannot be high enough when the rare earth oxide is prepared in this way.